STRUCTURE OF THE TEETH. 



105 



FIG. 48. 



composition, pierced by numerous fine canals dentine tubes 

 which radiate towards the surface, from the pulp cavity, in the 

 centre of the tooth. Processes 

 of protoplasm run in the den- 

 tine tubes from the tooth cells, 

 which line the pulp cavity and 

 preside over the nutrition of 

 the tooth. The cavity contains 

 vessels, nerves, p etc., which en- 

 ter at the root of the tooth 

 which is inclosed in a kind of 

 modified bone tissue called 

 crusta petrosa. 



The two rows of grinding 

 teeth, molars and premolars 

 (one on each side) of the lower 

 jaw are made to rub against 

 the corresponding teeth in the 

 upper fixed jaw by the com- 

 bined vertical and horizontal 

 movements induced by the ac- 

 tion of the powerful muscles 



of mastication, the temporal , Seot j on thl g h , f P ort ! on f the 



. . . fang of a tooth. (a) Dentine tubules 



muscles, together with the near the gurface of ^ fang; (ft) 



masseters and internal ptery- Granular layer ; (c) Crusta petrosa. 

 goids, all tending by their con- 

 traction to elevate the lower jaw and bring the teeth forcibly to- 

 gether. This action is opposed by the digastric, the geuio- and 

 mylohyoid muscles, which by their combined force depress the 

 jaw and separate the teeth. The horizontal movements are in 

 the main accomplished by the external pterygoid muscles, which, 

 acting together, pull the lower jaw forward so as to make the 

 lower teeth protrude beyond the upper. In this action they are 

 opposed by the digrastric and hyoid muscles. One external 

 pterygoid on either side acting alone, advances that side of the 

 lower jaw only, and thereby causes the lower teeth to incline 

 towards the opposite side in a lateral direction. The two muscles 



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